The present invention relates to a braking apparatus which works to prevent or restrain a relative movement between a rod and a given object that moves relative to the rod axially along the outer surface of the rod.
Pneumatic cylinders utilizing compressed air are known which comprise a braking apparatus that works to apply a braking force to a rod moving relative to a cylinder block so as to accurately position the rod at a desired position. Such a braking apparatus is disclosed in, for example, Japanese Utility Model Laid-open Publication No. SHO 59-13731 and Japanese Utility Model Publication No. HEI 1-38354.
FIG. 4 shows, by way of example, the general structure of the braking apparatus disclosed in the above-mentioned publications. The braking apparatus is composed of a full pneumatic brake mechanism, where compressed air is allowed to act within the apparatus in response to ON/OFF control of an electromagnetic valve, so as to control activation/deactivation of the braking function. General description on the braking apparatus will be given below with reference to FIG. 4.
The entire braking apparatus is cover ed with a cylindrical casing 61, through which a rod 22 extends within the braking apparatus. The casing 61 is mechanically secured to a movable object (such as a cylinder block in the case of a pneumatic cylinder) that is movable relative to the rod 22. Bearings 24L, 24R are provided at opposite ends of the casing 61 in such a manner that they are slidable on and along the rod 22 in the axial direction thereof. Packing members 62a, 62b and 63a, 63b are provided between the bearings 24L, 24R and the rod 22, so as to assure airtightness within the casing 61.
The casing 61 has first piping for supplying pressurized or compressed air from an unillustrated air pressure source to an air chamber 68 within the casing 61 via a pipe C4 and an electromagnetic valve 65, and second piping for supplying the compressed air from the air pressure source to air chambers 67L, 67R. In FIG. 4, the electromagnetic valve 65 is shown as being in the OFF state, so that the air chamber 68 is allowed to receive the compressed air supplied from the air pressure source via the valve 65 and first piping and the air chambers 67L, 67R are exposed to the external atmospheric pressure.
Brake pistons 69L, 69R are in contact with the casing 61 via packing members 71a, 70a and also in contact with the rod 22 via packing members 7b, 71b, in such a manner that the pistons 69L, 69R are slidable on and along the rod 22 in the axial direction thereof. The air chamber 67L is defined between the brake piston 69L and the casing 61, the air chamber 67R is defined between the brake piston 69R and the casing 61, and the air chamber 68 is defined between the brake piston 69L and the brake piston 69R.
In the air chamber 68 defined between the brake pistons 69L, 69R, a spacer member 68a is provided for reducing the volume of the chamber 68 to hold coil springs 72 within the chamber 68. The spacer member 68a is generally in the form of a circular plate whose outer peripheral portion is mechanically secured to the casing 61. The spacer member 68a has a central opening for passing therethrough the rod 22, and peripheral openings for holding coil springs 72 around the outer periphery of the central opening. The peripheral, i.e., spring holding openings are spaced from each other around the periphery of the rod 22 at angular intervals of 120 degrees, and they form a part of the first piping for supplying the compressed air to the air chamber 68.
The coil springs 72 are disposed in the respective spring holding openings for resiliently pressing the brake pistons 69L, 69R apart from each other along the axis of the rod 22.
Brake bushes 74L, 74R are also provided to surround the rod 22. As shown in FIGS. 5A to 5C, each of the brake bushes 74L, 74R is in the form of a cylinder which has a slit 51 defined along the full axial length thereof and other slits 52, 53, 54 each defined along a part of the axial length thereof. The inner peripheral surface of each of the brake bushes 74L, 74R is in contact with the outer peripheral surface of the rod 22. In addition, the inner peripheral surface of each of the brake bushes 74L, 74R is threaded, so that, when an external force is applied to the bushes 74L, 74R to radially press the rod 22, the inner peripheral surface in contact with the rod 22 causes a great frictional braking force. However, when the brake bushes 74L, 74R are in the normal state where no external force is applied thereto, the bushes 74L, 74R are only caused to slide along the outer peripheral surface of the rod 22.
Coned disk spring sections 73L, 73R are provided around the outer peripheral surfaces of the brake bushes 74L, 74R which include the slits 51 to 54. Each of the coned disk spring sections 73L, 73R comprises two coned disk springs 73, only one of which is shown in FIGS. 6A and 6B since they are identical in configuration. As clearly shown in FIGS. 6A and 6B, each of the coned disk springs 73 has a plurality of outer radial recesses 73a extending from the outer peripheral edge thereof and spaced apart from each other at equal angular intervals, and also a plurality of inner radial recesses 73B extending from the inner peripheral edge thereof and spaced apart from each other at equal angular intervals. The outer and inner radial recesses are arranged in an alternating fashion along the circumference of the coned disk spring 73. Further, in a sectional view taken along line A1-A2, each of the coned disk springs 73 has a bottomless-dish-like shape.
When the coned disk springs of the spring sections 73L, 73R are not pressed by the brake pistons 69L, 69R, the inner peripheral surfaces of the coned disk springs are held in close contact with the outer peripheral surfaces of the corresponding brake bushes 74L, 74R without leaving any gap and similarly the outer peripheral surfaces of the coned disk springs are held in close contact with the inner peripheral surfaces of the corresponding brake pistons 69L, 69R without leaving any gap, as shown in FIG. 7A. When the coned disk springs of the spring sections 73L, 73R are pressed by the brake pistons 69L, 69R, the springs are deformed in such a manner that the inner diameter D1 of each of the coned disk springs is caused to become smaller. Thus, a force is applied to radially inwardly press the brake bushes 74L, 74R with the slits 51-54 of the bushes 74L, 74R considerably reduced in width. Because of this, the inner diameters of the brake bushes 74L, 74R are caused to become smaller so that the threaded inner peripheral surfaces of the bushes 74L, 74R are brought into biting engagement with the rod 22, with the result that the braking apparatus is brought into the braking (active) state.
As soon the brake pistons 69L, 69R stop pressing the spring sections 73L, 73R, each of the coned disk springs resiliently reverts to its original shape so that the pressing force from the spring sections is no longer applied to the brake bushes 74L, 74R. This causes the inner peripheral surfaces of the brake bushes 74L, 74R to separate from the rod 22, with the result that the braking apparatus is brought into the non-braking (inactive) state.
When the unillustrated air pressure source is in the OFF state, the braking apparatus is always maintained in the braking state through the resilient force of the coil springs 72. Namely, unless the braking apparatus is supplied with compressed air from the air pressure source, it can be maintained in the braking state (i.e., self-locking state).
When the air pressure source is in the turned-ON state, the braking function of the braking apparatus can be activated or deactivated through the ON/OFF of the electromagnetic valve 65. When the electromagnetic valve 65 is in the OFF state as shown in FIG. 4, the compressed air is introduced from the air pressure source into the air chamber 68 while the air chambers 67L, 67R are both exposed to the external atmospheric pressure. Thus, the brake pistons 69L, 69R which are normally pushed away from each other by the coil springs 72 are even more strongly pushed apart from each other by additional high pressure of the compressed air. Accordingly, the coned disk spring sections 73L, 73R are pressed against the inner surface of the casing 61 in the axial direction of the rod 22, with a greater force than when the air pressure source is in the turned-OFF state. Due to this, a great frictional braking force is caused between the brake bushes 74L, 74R and the rod 22, and the braking state of the braking apparatus can be made stronger and more reliable.
On the other hand, when the electromagnetic valve 65 is in the ON state, the compressed air is introduced from the air pressure source into the air chambers 67L, 67R while the air chamber 68 is exposed to the external atmospheric pressure. Thus, high pressure of the introduced compressed air acts to reduce the resilient force of the coil springs 72, so that the brake pistons 69L, 69R are moved toward each other to compress the coil springs 72. This eliminates the pressing force applied to the spring sections 73L, 73R and thus each of the coned disk springs resiliently reverts to its original shape, so that the brake bushes 74L, 74R disengage the brake rod 22. As the result, the braking apparatus is brought into the non-braking state where the casing 61 is allowed to freely move along the rod 22.
The above-mentioned prior art braking apparatus is relatively superior in performance among various braking apparatus of the type where a braking force is applied to a rod, since it can provide a sufficiently great braking force and has good response characteristics.
But, the prior art braking apparatus mentioned above presents a problem that, as its braking action is performed repeatedly, for example, up to a million of times, considerable abrasion may occur in the outer and inner peripheral edges of the coned disk springs, thus deteriorating the braking capability of the apparatus.
Namely, before the braking action has not been performed many times, the coned disk spring engages with the corresponding brake piston and brake bush in a manner as shown in FIG. 7A. As the brake piston moves to the left as viewed i n FIG. 1, the coned disk spring is deformed as shown in FIG. 7B to press the brake bush radially inwardly. This causes the brake bush to be pressed radially inwardly against the rod 22 with a pressing force .DELTA.L1 that equals L1 (sin .theta.b-sin .theta.a). Namely, this pressing force L1 corresponds to the length of a diagonal line L1 in the rectangular sectional shape of the coned disk spring and to the angles .theta.a, .theta.b defined between the diagonal line L1 and the outer surface of the brake bush.
However, as the braking action is performed repeatedly, for example, up to a million of times, considerable abrasion may occur in the outer and inner peripheral edges of the coned disk springs so that the length of the diagonal line becomes smaller as shown by L2 in FIG. 7C. Because of this, the diagonal line L2, even when the braking apparatus is not in the braking state, formes an angle .theta.c that is greater than the angle .theta.a of FIG. 7A. Accordingly, even when the brake piston moves to the left so that the angle .theta.b is reached as shown in FIG. 7D, the brake bush is pressed against the rod 22 only with a pressing force A.DELTA.L2 (=(sin .theta.b-sin .theta.c) which is smaller than the above-mentioned force .DELTA.L1. As the result, the brake bush fails to contact the rod to a sufficient degree, and hence a sufficient braking force can not be provided by the braking apparatus.